It is likely that there are many causes of retinal disorders, among which are those associated with genetic or acquired defects in expression of photoreceptor cell proteins. Mild, well-tolerated disorders include several known hereditary defects in photopigments associated with color- blindness. Some forms of hereditary blindness known as retinitis pigmentosa have now also been attributed to photopigment defects. The fruit-fly, Drosophila melanogaster is susceptible to a blinding mutation in a gene called norpA (no receptor-potential a-wave) which codes for an enzyme called phospholipase C (PLC) expressed in photoreceptor cells. We have now identified a close relative of the norpA gene in bovine retinal complementary DNA (cDNA), at transcript levels possibly one five hundredth of the level of rhodopsin. The bovine gene, like the Drosophila gene, codes for PLC, an enzyme likely to be central to vertebrate photoreceptor cell calcium flux and to adaptation or recovery. The proposed research will (I) localize gene products of norpA-like and other PLC isoforms identified in retinal complementary DNA; (II) identify or construct full-length cDNA clones which code for photoreceptor PLCs; (III) improve or supplement present purification methods using PLC- specific immunoaffinity adsorption; (IV) compare PLC isoforms by primary structure, localization and expression levels of mRNA and protein; (V) express selected full-length clones in appropriate cellular or cell-free expression systems, and compare recombinant with native proteins by structural and enzymatic methods; (VI) examine regulatory mechanisms of each recombinant PLC in vitro by reconstitution with candidate regulatory elements including G proteins, rhodopsin, arrestin, selected phospholipids and kinases. PLC genes are new candidates for inherited forms of human retinal blindness, and retinal PLCs may also be unintended targets of a number of psychiatric drugs.